In some satellite and terrestrial mobile communication systems, terminals are available which provide both voice and data communication. For example, some GSM mobile telephones are connectable to data terminals, such as portable computers. During a data call using such mobile terminals, TDMA slots are assigned to the data call in a similar way as to a voice call, so that a constant bandwidth connection is established both in the forward and return direction. Mobile satellite systems, such as the Inmarsat-B™, Inmarsat-C™ and Inmarsat-M™ satellite communication systems, allow fixed-bandwidth data communications.
Such systems are suitable for the exchange of some types of data, but are primarily designed for voice communications, which require a constant symmetrical data rate. Such systems are not optimized for both data and voice communication.
There is an increasing demand for data communications which require intermittent bursts of data to be sent or received at a high data rate, while requiring only low data rate communication at other times. For example, it is desirable when using a Web browser for requested pages to be downloaded as quickly as possible to the user terminal, but little or no bandwidth is required in the forward direction while the user reads the downloaded page. Such usage is also very asymmetrical, since the user only needs to send requests for new pages or small amounts of data in the return direction. The use of such applications over the GSM system is unsatisfactory, since for some of the time during the data call the allocated bandwidth is not used, but while large amounts of data are being downloaded, the allocated bandwidth is insufficient.
Furthermore, it would be desirable to allow multiple calls to be handled concurrently by the same mobile terminal. For example, during a telephone call, a mobile user may wish to refer to data from an online database, or receive an incoming facsimile.
A variable data rate satellite communication system is described in U.S. Pat. No. 4,256,925. In this system, a satellite is used for communication between a plurality of ground stations. Each ground station requests a proportion of the total channel capacity in accordance with the traffic load of voice and data call to that ground station. A reference station in the network allocates the channel capacity among the stations.
An ATM (asynchronous transfer mode) satellite communication system is described in U.S. Pat. No. 5,363,374. Each earth station requests sporadic connection to the communication system and a central management station determines whether to accept or refuse the connection according to the available bandwidth.
The document GB 2 270 815 A discloses a cellular mobile radio system which provides a packet reservation multiple access protocol for implementing a variable bit rate service. The mobile terminals contend for access in the same time slots in which they will transmit information. In order to support variable bit rates, the terminals have the ability to reserve multiple slots in any given frame. Both voice and data calls may be supported in the same frame, with a larger burst size being used for data traffic for greater efficiency, and a small burst size being used for voice traffic to reduce the delay incurred in waiting for enough information from a voice code.
The document EP 0 713 347 A discloses a system for transmission of STM and ATM traffic on a broadband communications network, such as a fiber/coax network or a wireless network in which mobile stations depend on a base station for feedback. ATM calls can be constant bit rate, delay sensitive variable bit rate, delay tolerant variable bit rate or contention based. Delay sensitive variable bit rate calls are allocated time slots in accordance with a statistically weighted incremental bandwidth determination that takes into account existing variable bit rate traffic and the statistical characteristics of the new call request. ATM traffic is assigned a minimum guaranteed bandwidth and any spare bandwidth is assigned among existing ATM calls or used to admit a new ATM call.